As part of a broad program to study aerobic energy metabolism in parasites and mammals, current interest is focused on the role of flavoproteins as electron carriers in the respiratory chain of Entamoeba histolytica. Flavins were identified by difference and fluorescent spectrophotometry; oxygen consumption was determined polarographically. Acid extracts of sonicated ameba revealed a flavin content of 0.10 micromoles per gm of protein. Extracts of the respiratory enzyme NADPH oxidase (diaphorase) had a flavin content of 0.60 micromoles per gm protein. A unique feature of the amebal flavin is that virtually all of it is acid-extractable, whereas in higher organisms substantial portions of the flavin remain bound to the acid-extracted residue. These findings, together with our previous data, show that the diaphorase contains 6 moles of non-heme iron, 6 moles of acid-labile sulfide and 1 mole of flavin per gm protein. Mammalian studies initiated this year included a collaborative investigation on the effect of inhibitors of oxidative phosphorylation on the release reaction in bovine adrenal medulla chromaffin granules. This work supports the hypothesis that such uncouplers act as proton ionophores, and contribute significantly to our knowledge of bioenergetics. BIBLIOGRAPHIC REFERENCE: Weinbach, E.C., Harlow, D.R., Claggett, C.E. and Diamond, L.S.: Entaboeba histolytica: Diaphorase activities. Exp. Parasitol. 41: 186-197 (1977).